Thanks for the very quick reply. The subdomain seems like an excellent idea to me, although using the IP address isn't a problem - it's what I have to do at the moment.
One more question. Is it possible for me, through the reseller account, to restore cPanel full backups taken from the other server? I know you can do the migration, but I'd rather take my time and do it one site at a time and check everything is working on one site before moving the next one, so it would be easier to do it myself. It's not that big a deal to upload the pages and recreate the databases manually, but it would save a bit of time if I could just upload and then restore a backup.